1. Field of the Invention
The present invention relates to 3-aryl-3-methyl-quinoline-2,4-diones acting as a 5-HT6 receptor antagonist, a preparation method thereof, and a pharmaceutical composition containing the same for treatment of central nervous system disorders.
2. Description of the Prior Art
Although the function of serotonin (5-HT) in the central nervous system is still being clarified, various studies have indicated 5-HT has been implicated in the aetiology of many disease states and may be particularly important in mental illness, such as depression, anxiety, schizophrenia, eating disorders, obsessive compulsive disorder (OCD), migraine and panic disorder. Recent advances in pharmacology, molecular biology, and genetics on the serotonin system hold out the promise of the development of improved pharmacological treatment for some aspects of neurological diseases. Indeed, many currently used treatments of these disorders are thought to act by modulating serotonergic neurons. During the last decade, multiple 5-HT receptor subtypes have been characterized. Initially, receptor subtypes were characterized using pharmacological tools only. On the basis of the receptor binding profiles, common secondary messenger coupling and the functional activity of ligands, four main subgroups of 5-HT receptors, termed 5-HT1, 5-HT2, 5-HT3 and 5-HT4, were identified. More recently, molecular biological techniques have both confirmed this classification, in that each subgroup has been found to have relatively dissimilar protein structures, and led to the identification of novel 5-HT receptors (5-HT1F, 5-HT5, 5-HT6 and 5-HT7) enabling them to be cloned, expressed in cultured cell lines [Hoyer, D. et al., Pharmacol. Biochem. Behav. 2002, 71, 533-554; Kroeze, W. K. et al., Curr. Top. Med. Chem. 2002, 2, 507-528].
Most recently, the 5-HT6 receptor has been cloned from rat cDNA based on its homology to previously cloned G-protein-coupled receptors. The rat receptor consists of 438 amino acids with seven transmembrane domains and is positively coupled to adenylyl cyclase via the Gs G-protein [Monsma, F. J. et al., Mol. Pharmacol. 1993, 43, 320-327]. Human 5-HT6 receptors, a 440 amino acid polypeptide, display 89% overall sequence homology with the rat receptors and is positively coupled to an adenylate cyclase second messenger system [Kohen, R. et al., J. Neurochem. 1996, 66, 47-56]. Rat and human 5-HT6 mRNA is located in the striatum, amygdala, nucleus accumbens, hippocampus, cortex and olfactory tubercle, but has not been found in peripheral organs studied. In pharmacological studies, tritiated 5-HT, tritiated LSD, and [125I]-2-iodo LSD have been used to radiolabel 5-HT6 receptors. 5-HT binds with moderately high affinity (Ki=50-150 nM). Tricyclic antipsychotic agents and some antidepressants bind with significant affinity. A related investigation examined antipsychotics in greater detail and found that representative members of several classes of antipsychotics bind with high affinity. Examples include phenothiazine chlorpromazine, thioxanthene chlorprothixene, diphenylbutylpiperidine pimozide, heterocyclic antipsychotic agent loxapine and clozapine [Roth, B. L. et al., J. Pharmacol. Exp. Ther. 1994, 268, 1403-1410]. These results led to suggestions that 5-HT6 receptors might play a role in certain types of psychoses and that they might represent significant targets for the atypical antipsychotics in particular.
Until selective ligands were developed, exploration of 5-HT6 pharmacology was largely dependent on the use of nonselective agents. In the absence of selective ligands for the receptor, functional studies have been carried out using an antisense approach. 5-HT6 specific antisense produced a specific behavioural syndrome of yawning, stretching and chewing, but had no other discernable action in rats. The non-selective ligands were useful for investigating the pharmacology of 5-HT6 systems in preparations where other 5-HT receptors were absent (e.g., cAMP assays) ; however, owing to their lack of selectivity, they were of limited value for most other pharmacological studies.
Recent advent of selective agents has greatly benefited 5-HT6 studies, and this field of research has recently exploded. The development of more selective ligands may therefore lead to treatments with increased efficacy and reduced side effects. Alternatively, selective ligands may form completely novel therapies. It was not until 1998 that the first 5-HT6-selective antagonist was described, and this prompted others to quickly report their efforts in this area. Sleight et al. at Hoffman-La Roche Co. identified the bisaryl sulfonamides Ro 04-6790 (1, Ki=55 nM), Ro 63-0563 (2, Ki=12 nM) as very selective 5-HT6 antagonists [Sleight, A. J. et al., Br. J. Pharmacol. 1998, 124, 556-562]. Shortly thereafter, MS-245 (3, Ki=2.3 nM) was reported. Interestingly, although they represented independent discoveries, all three were identified by random screening methods and all three possess a sulfonamide moiety.
One problem associated with these antagonists was their low penetration of the CNS. At the time, Smith-Kline Beecham Co. also pinched out compound 4 via high-throughput screening. It displayed high affinity (Ki=5 nM) for 5-HT6 receptors and >50-fold selectivity over 10 other 5-HT receptors and no measurable affinity for 50 other receptor/binding sites. It was a pure antagonist of cAMP accumulation (pKb=7.8) [Bromidge, S. M. et al., J. Med. Chem. 1999, 42, 202-205]. It was moderately brain penetrant (25%) but subject to rapid blood clearance resulting in low bioavailability.
An ensuing structure activity study identified SB-271046 (5, Ki=1 nM; >200 selectivity over 50 other receptors) retained antagonist activity, and although less brain-penetrant (10%), it showed excellent (>80%) oral bioavailability.

Subsequent studies by this group showed that SB-357134 (6, Ki=3 nM) with a low clearance rate and excellent oral bioavailability. In 1999, Glennon et al. undertook a structure affinity investigation of the binding of tryptamine derivatives at human 5-HT6 receptors [Glennon, R. A. et al., J. Med. Chem. 2000, 43, 1011-1018]. MS-245 was found as an antagonist (pA2=8.88) with high affinity (Ki=2.3 nM). In contrast to the above-mentioned sulfonamides or tryptamine derivatives, Hoffmann-LaRoche (7) and Pharmacia-Upjohn (8, Ki=1.4 nM) recently revealed several sulfones [Slassi, A. et al., Expert Opin. Ther. Pat. 2002, 12, 513-527]. Newer agents continue to be developed in attempts to improve pharmacokinetic and pharmacodynamic properties. Now that some tools are available, attention is focusing more and more on the function of 5-HT6 receptors.
A typical antipsychotics, in particular, display high affinity at these receptors (vide supra). In addition, the tritiated atypical antipsychotic agent [3H]clozapine was shown to label two populations of receptors in rat brain and one population was thought to represent 5-HT6 receptors [Glatt, C. E. et al., Mol. Med. 1995, 1, 398-406]. Vogt et al. performed a systematic mutation scan of the coding region of the 5-HT6 receptor gene of 137 individuals (including schizophrenic and depressed patients) and concluded that the gene might be involved in bipolar affective disorder [Vogt, I. R. et al., Am. J. Med. Genet. 2000, 96, 217-221].
Prior to the identification of 5-HT6-selective agents, Bourson et al. demonstrated that intracerebroventricular administration of antisense oligonucleotides produced in rats a specific behavior of yawning, stretching, and chewing, which could be antagonized by atropine [Bourson, A. et al., J. Pharmacol. Exp. Ther. 1995, 274, 173-180]. Sleight et al. demonstrated that Ro 04-6790 (1) was capable of inducing this same effect. Owing to a relationship between cholinergic function and cognition, this led to speculation that 5-HT6 receptors might be involved in memory and cognitive dysfunction [Sleight, A. J. et al., Neuropharmacology 2001, 41, 210-219; Rogers, D. C. et al., Psychopharmacology (Berlin) 2001, 158, 114-119].
In addition, because antisense oligonucleotide pretreatment and Ro 04-6790 administration both led to decreased food intake by rats, it was suggested that 5-HT6 receptors might be involved in the regulation of feeding. Furthermore, Russell and Dias have questioned the postulate that 5-HT6 antagonists increase cholinergic transmission [Russell, M. G. N.; Dias, R., Curr. Top. Med. Chem. 2002, 2, 643-654].
Despite the mechanistic disagreement, there is evidence for the involvement of 5-HT6 receptors in learning and memory. When a water maze was used with rats as subjects, SB-271046 (5) and SB-357134 (6) showed significant improvement in retention of a previously learned task. Furthermore, SB-271046 (5) increased extracellular glutamate levels in frontal cortex and dorsal hippocampus by several fold, leading to the conclusion that selective enhancement of excitatory neurotransmission by SB-271046 supports a role for 5-HT6 receptor antagonists in the treatment of cognitive disorders and memory dysfunction [Dawson, L. A. et al., Neuropsychopharmacology 2001, 25, 662-668].
In addition, SB-357134 (6) produced a potent and dose-dependent increase in seizure threshold (rat maximal electroseizure threshold) following oral administration, suggesting possible therapeutic utility in convulsive disorders [Stean, T. O. et al., Pharmacol. Biochem. Behav. 2002, 71, 645-654]. These findings are consistent with an earlier finding that SB-271046 (5) and Ro 04-6790 (1) possess anticonvulsant activity.
Overall, there is some evidence to suggest that 5-HT6 receptors could be involved in psychosis. There is still more evidence that these receptors are involved in cognition and learning and additional evidence that they might have a role in convulsive disorders and appetite control. Although additional studies are certainly warranted, particularly with some of the newer 5-HT6 antagonists that are more brain-penetrant than the earlier agents, the future of 5-HT6 receptor ligands as potential therapeutic agents is quite exciting.
The inventors made an effort to develop a 5-HT6 antagonist having excellent binding affinity and selectivity, and has completed the present invention by discovering that quinoline-2,4-dione derivatives are 5-HT6 antagonists having very excellent binding strength and selectivity compared to sulfonamide or sulfonic structures disclosed in the prior art.